1. Field of the Invention
The present invention relates to a radar altimeter having a static accuracy circuit.
2. Prior Art
Radar altimeters, also often called radio altimeters, are used in aircraft to give the pilot an indication of his altitude above ground level (AGL) as opposed to the aircraft's barometric altimeter which is set to display the height above Mean Sea Level (MSL). Generally radar altimeters are used as a landing air or in ground proximity warning equipment. Therefore, the range of measurement is generally limited to 2500 feet; although many commercial air carriers only display radar altitude to the pilot when the aircraft is below 500 feet AGL.
Frequency modulated/continuous wave (FM/CW) radar altimeters are so named because they transmit a CW signal that is swept in frequency as opposed to a pulsed signal. The frequency sweep can have any number of shapes but a linear triangle or sawtooth is the most common.
The distance to the ground level, or the AGL altitude, is determined by measuring the frequency difference between the current transmitter frequency and a prior transmitter frequency delayed by the time required for the signal to propagate to the ground and back.
These prior art radar altimeters suffer from erratic performance when the altimeter's platform is nearly stationary over the ground. This is not a concern in fixed wing aircraft and has led to a problem largely being ignored. However, stationary operation is a condition common to helicopters prior to landing or in a sustained hover. In particular, the growing use of helicopters for medical evacuation where night operations at uncontrolled landing sites are a must has provided impetus for this invention.
The source of the erratic performance while the altimeter's platform is stationary can be traced to the nature of the ground return signal and is the same for both pulse and FM/CW radar altimeter implementations. The composite ground return signal is composed of a myriad of reflections of various amplitudes, phases, and distances which are dependent upon the radar altimeter antenna patterns, the frequency, the terrain, and the motion of the radar altimeter. The composite return signal is thus formed from the sum of these many components. Under certain circumstances the many components of the composite return signal cancel themselves out which leads to the erratic performance. This cancellation effect is frequency dependent and has a bandwidth that is generally less than the band swept by an FM/CW altimeter. By the utilization of this principal, the erratic performance can be overcome in FM/CW altimeter designs by the method of this invention.
Considerable insight into the return signal cancellation phenomenon is obtained by considering that a change in distance to the reflection has the same effect on the return signal as a change in transmitter frequency. Since the cancellation effect disrupts the altimeters performance over very narrow bands of altitude then a change in the transmitter frequency by an equal percentage will be sufficient to remove the cancellation.
In an FM/CW altimeter the transmitter frequency is constantly sweeping over a band of frequencies thus insuring that the return signal cancellation effects only a portion of the sweep.
The effect on measured altitude is multiplied by the aircraft installation delay used to allow altimeters to operate to zero altitude. For example, a prior art altimeter processor might measure an internal altitude, including the aircraft installation delay, 25% low when the signal cancellation extends over 25% of the transmitter sweep. However, the error in displayed altitude would rise to 50% if the actual altitude is equal to the aircraft installation delay. Since the aircraft installation delay is typically 20 feet to 40 feet, and this is the range of altitudes where the return signal cancellation phenomenon is most often encountered, the aircraft installation delay often acts to compound the erratic performance .